Our invention relates to cathode ray tubes (CRTs) in general and, in particular, to those of the type having an electron lens system for amplifying, in coaction with a postaccelerating or intensifier electrode, the deflections of the electron beam in both horizontal and vertical directions. More particularly, our invention concerns improvements in the electron lens system of the kind described and claimed in U.S. Pat. No. 4,302,704 filed by Saito and assigned to the assignee of the instant application. The improved lens system of our invention, herein referred to as the scan expansion lens system, finds use in oscilloscope and storage CRTs, among other applications.
The scan expansion lens system according to the above cross referenced U.S. Pat. No. 4,302,704 comprises three boxlike lens elements or electrodes arranged end to end, with insulating gaps therebetween, so as to encompass the trajectories of the electron beam from the deflection system to the target of the CRT. The target side electrode of this prior art lens system has its beam exit end closed by an end plate having defined therein an aperture or slot elongated horizontally. So constructed, the conventional lens system provides a divergent electron lens for the beam that has been deflected horizontally, and two successive convergent electron lenses for the beam that has been deflected vertically, one of the two convergent lenses being produced at and adjacent the end plate aperture of the target side electrode by the field of the postaccelerating electrode. The apertured end plate, which distinguishes this known lens system from the more conventional ones such as disclosed, for example, in U.S. Pat. No. 3,496,406 to Dechamps, has proved to be of immense utility for the attainment of high deflection sensitivity without spot defocusing and the improved linearity of the deflection factor in a vertical direction.
We have found, however, a weakness in this prior art scan expansion lens system. Its end plate aperture is of constant width or vertical dimension throughout its horizontal dimension. Consequently, the horizontal potential distribution at and adjacent this end plate, due to the potential difference between the target side electrode of the lens system and the postaccelerating electrode, is so linear that it serves little or no purpose for the magnification of horizontal beam deflection. The utility of the lens system will certainly be still more enhanced if its end plate aperture works also for the improvement of linearity in the horizontal deflection factor.
A conventional solution to this problem is found in Japanese Laid Open Patent Application No. 56-152143 laid open to public inspection on Nov. 29, 1981. This prior Japanese application suggests an end plate aperture gradually increasing in width from the midpoint to its horizontal dimension toward both ends. The solution has proved unsatisfactory, however. The end plate aperture of the above modified shape can amplify horizontal deflection only at the sacrifice of the linearity of the horizontal deflection factor.